In the production of extruded foil tubes it is necessary to use extruders with rotating or oscillating heads to ensure that the extruded tube is of uniform wall thickness and, thus, uniform strength. A further requirement is pressurizing the tube to form a bubble therefrom at the mouth of the extruder, the size of the bubble determining the diameter of the tube. Generally, upon cooling of the portion of the tube which forms the bubble, this cooling being at least partially implemented with the aid of an external air source blowing air on the outside of the bubble, the tube is collapsed therefrom to form a folded, flattened tube or two-ply web which is transported away from the extruder station for further working, such as cutting and heating to form bags.
Owing to the motion of the extruder head, the feeding of air to the tube is particularly problematical. Air losses from the tube are, in most cases, slight or marginal, it only being necessary to occasionally feed air to or draw it from the tube. In other cases, such as when the tube is extruded with integral fasteners (e.g. of the zip-lock type), continuous large air losses arise in the bubble. In these cases an air-loss compensation must be finely and continuously implemented.
In a known extruder assembly an annular channel or ring fastened to the extruder head engages another ring fixed to the extruder body, this ring being connected to a stationary air-supply system including a blower. The head is provided with an outlet communicating with the inside of the tube and with an air chamber formed by the pair of rings and charged with air from the blower. Because of the difficulty of obtaining a satisfactory seal between the two rings, air losses in a system of this sort are high. The length of the air supply line, together with the leaking air chamber, render the system sluggish and unresponsive to changes in power output of the blower, whereby a fine control of tube pressure is difficult to attain. Further disadvantages include bulkiness due to the at least partially overlapping rings and an exhorbitant power draw for the blower due to the large air losses in the system.